After Pentatonix won the third season of NBC-TV’s a cappella singing competition “The Sing-Off” in 2011, it spent significant time — and a couple of albums — chasing the latest hits, doing voice-only renditions of current chart-toppers.

But it was a disc of completely original songs that gave Pentatonix its first No. 1 album in 2015.

Its second No. 1 disc came a year later, in October, with “A Pentatonix Christmas,” a more traditional disc of Christmas favorites.

Now the quintet has done the even more obvious: In April, it released an EP, “PTX, Vol. IV — Classics,” that digs into the songbook of classic rock, country and even deeper classics, mining songs such as “Over the Rainbow” and The Andrews Sisters’ World War II hit "Boogie Woogie Bugle Boy.”

While the disc didn’t hit No. 1, but became the group’s second-highest-charting album of non-Christmas music, peaking at No. 4.

It also keeps Pentatonix at the forefront of a cappella music, which not only has had resurgence in recent years with the success of shows such as “The Sing Off” and hit movies such as the “Pitch Perfect” series, but also has proven surprisingly resilient.

It’s been a decade since a cappella stepped back into the mainstream with Straight No Chaser’s viral YouTube cover of “The 12 Days of Christmas” mashed with Toto’s “Africa.”

In a recent telephone call from “somewhere on the open road between Nashville and Columbus, Ohio,” Pentatonix member, Kevin Olusola, the group’s vocal percussion, beatboxing, backing vocals and cello, talked about the group’s career, its new disc, and its future.

Here’s a transcript of the call:

LEHIGH VALLEY MUSIC: Hey Kevin, how are you?

KEVIN OLUSOLA: “Hi, John, how are you?”

Just fine, thanks. Thanks for taking out the time to speak with me today.

“Absolutely.”

You’re playing our Allentown Fair on Sept. 2.

“Yeah, I’m very excited for that one – especially ‘cause I think my parents are coming to that one.”

Really? Where are your parents from?

“Well, my parents live in Cambridge, Maryland.”

Oh, OK. That’s not too far. So let’s jump into this. And I’ll start with the news. Tell me about the decision to release [the cover of Robyn’s] “Dancing on My Own.” What made you guys do that song and put it out as a single?

“Well first of all, we just love that song, and we love Robyn. And that song is so gorgeous, and we knew we do it well. And we knew a big decision just happened with our band – Avi [Kaplan] will be moving on. But we still love what we do – everything is really wonderful, and we’re going to continue making music for you guys.

“So this is kind of to show people that we’re continuously thinking about that, thinking about the fans, and we want to make music that they’ll continue to love.”

And I want to talk a little bit later about Avi’s decision. But I also wanted to ask about PTX Vol. IV, the last album that you guys put out. What were you looking to do, and why did you choose to include more rock and classic pop on it?

“Well, it’s funny. Throughout our whole career, people have always asked us to do that.”

Oh really?

“When we were on ‘The Sing Off,’ we did Janis Joplin and we also did Steppenwolf ‘Born to Be Wild.’ And people really loved those and always asked, ‘Man, will you guys ever think about doing a full album of cover songs of just classics?

“And we thought, ‘This will be a really good time to do it. We’ve always chased after the next newest, biggest song, and we thought, ‘Well, we’ve built our fan base; we think they would really appreciate something like this. And we’re so happy that we did it. It was a fun record to put out.”

How did you guys decide what you were going to include? I mean, whose songs … who made the decision about the songs?

“Every single decision we make has to be a group decision. So honestly, we got to [members] Scott [Hoying] and Mitch’s [Grassi] house, where they live, and we just started to pick songs that we really loved. And we would talk about how we could arrange it, possibly, and if something really clicked with us, we would do that.

“So for example, I think Scott and Mitch were talking about ‘Take On Me,’ and I heard it, and I said, ‘Oh, wouldn’t it be cool to do something energetic,’ and everybody was really behind it. It just happened to be very … it was very informal, but everybody’s opinions, really, I think, counted.”

OK, now let’s talk a little bit about Avi’s decision. What’s the reasoning behind it, and what’s the group going to do?

“Well, first of all, being on the road for as long as we have – and we’ve been on the road every single year, we’ve put out one or two albums every single year. So the pace of our lives is really crazy – and especially because we knew we’re somewhat of a novelty group in the pop industry, we knew we had to work that much harder.

“And it can take a toll on people. It really, really does. And Avi is very, very close to his family and he want to be there for them. And he also has music that he’s always wanted to do, of his own. I just think it was time for him to step out on his own and do his own thing, and live life on his own terms so that he can kind of be able to take care of himself a lot more. And just lead his own life.

“And we’re really proud of him for that, ‘cause that’s a very difficult decision, especially with all the success that we’ve had. I’m very, very happy that he’s taking the time to do that.”

And has the band decided what it’s going to do? I mean, are you going to find a new member? Are you going to perform as a quartet?

“We’re still figuring that out. Most likely it’ll be – Pentatonix the brand will be the four of us. That’s what we’re thinking, but we’re still trying to figure that out. But, I think we’re going to definitely audition bass singers because you can’t do what we do with just four people. But, you know, we’re figuring it out.”

OK. When you were talking about the life of the band – it really has been quite a ride for you guys for the last, whatever, six years. You win “The Singoff,” you win three Grammys=, your records go platinum and gold. What does that feel like? How does that feel, that so much has happened in such a short period?

“Oh my gosh. It’s in a whirlwind. We to begin? I mean, when we started off, we just did this because we loved it. I mean, we were – we got together for a TV show, and even after the TV show, we said, ‘You know what? We just love what we do. Sure, the odds are extremely high, but at the end of the day we get to love what we do, and if we touch one other person with what we do, we’ve done our job.

“But with the kind of success we’ve had, it’s unprecedented in a cappella. And we’re just – we take it one day at a time, and I think this year especially has allowed us to take time off and really reflect on what we’ve done. ‘Cause when we take time to reflect, it’s truly incredible.

“We can watch videos of our whole journey – from old tours to doing the AMAs [American Music Awards] in 2013 and through the ‘Star Wars’ medley or when we sang with Stevie Wonder on the Grammys. I just sit back and say, ‘I can’t believe we did all this!’

“So the reflection’s been well necessary – very, very necessary.”

Your last two albums have hit No. 1. What do you think made them so special? Or pushed them to the top of the charts?

“Um, wow. Well, I think with the original album – people were ready for that. I think we were ready to put out an original album, and I think our fans were really ready for that as well. And, you know, we were neck and neck with Demi Lovato trying to hit No. 1, and we fought tooth and nail to get there.”

[Laughs]

“I mean, I can’t tell you the type of conversations that our label had, were they said, ‘I will get this to No. 1.’ And the conversations we … I mean, we were calling every radio station ourselves. In the RV office – we would tell them what we were trying to do and how this can be a historic moment in a cappella.

“And so we really fought tooth and nail for that, and our fans really came through. And the community – in terms of social media that we had. They came out and really tweeted about it, so it was really wonderful to get so many of our friends to root for us.

“Christmas and a cappella – what we learned really, really well is that they go extremely well together. People love what we do during Christmas, and a cappella does well there. I mean, you look at Straight No Chaser, they do so well during Christmas time. And we thought the way we do a cappella, the way we do Christmas, people really took to it, and I think that’s what made us go No. 1.

“And I think it’s funny – usually an album goes to No. 1 at first, and then it trickles off. But “ That's Christmas to Me” just continuously ramped up, ramped up, ramped up in the charts until it got to No. 1 the last week in December.

“So we were really excited about that. And it made going into the new year that much sweeter.”

It’s interesting that you mentioned Straight No Chaser because Jerome Collins, the lead singer, is from Allentown – and they’re sort of our hometown band. And Walter Chase is from the next city over – Easton. So you guys are going to be coming into the hotbed of Straight No Chaser when you play at Allentown Fair.

“Oh, that’s awesome! What a cool fun fact.”

And that brings me to a larger question: When you talk about a cappella and the growing popularity of it, to what do you attribute that? Why do you think a cappella is catching on -- or has caught on – the way it has?

“Uh, I think so many things have happened in the mainstream that definitely brought awareness and attention to a cappella. The ‘Pitch Perfect’ movies, ‘The Sing-Off’ – I mean, the college a cappella scene definitely has become really hot, which is definitely wonderful.

“And I think people, especially in today’s music scene, I think people are really wanting something … a lot more organic. That’s not so heavily produced. And don’t get me wrong – I love produced music. If you look at all the solo albums everybody in the band has put out, it’s produced music – because we all love it.

“It’s just that we think because of that, we think there is a void that is missing when it comes to organic music, and we think that we can fill that void.”

Those were the questions I had prepared. Is there anything else you’d like to see in my story?

“Man, I am so excited to come to Allentown. And y’all get ready to rock, man. Get ready to rock!”

PENTATONIX, with Us The Duo,7 p.m. Sept. 2, Allentown Fair, 17th and Chew streets. Sold out; tickets available on secondary markets, www.allentownfairpa.org, 800-745-3000